This project deals with the molecular details of messenger RNA biosynthesis -- the definition of transcription units in mammalian cells, the correspondence between these transcription units and mRNAs formed from them -- and the potential steps along this complicated pathway where regulation of mRNA supply is effected. The use of recombinant DNA clones and specific segments of Adenovirus DNA has allowed the suggestion that regulation occurs both at the level of transcription and mRNA stabilization.